Hi, im not new to aquarium hobby, but new to goldfish, i now have some oscars in main tank, and i had a 20 gallon tank not in use so i thought i set it up with some goldfish, 3 of them, they look good and are active.The backside of tank is painted black so the orange color realy shows.but one of the fish have some black at the end of lower and top finns so that kind of disapears in to the background hehe, any idea if that will disapear when he grow? or will it always have that black color on tip of the finns, the rest of the body is orange.and is it comet?

Yea , they do look like comet goldfish. All of them look very healthy with nice erect fins with no sign of finrot. They look like they're only a few months old.

The black markings could be the result of ammonia burns while the fish were at the pet shop or in your current tank. To be sure, always check your water parameters for spikes and perform partial water changes if necessary to bring these levels back to normal. If you're unfamiliar with cycling an aquarium , you can do a google search and you will find many good resources to read about it. If the black markings were caused by the ammonia (or nitrite) spikes , they will eventually heal and the original coloration will return.The black markings could also be a part of its natural coloration.

If you know your water parameters (ammonia, nitrite , nitrate, pH) , you can post it here , and we'll help you determine if your water is safe for the fish.

i dont have anything to check my water parameters but tank have been upp running for around a month so it should be fine, and the fluval 205 have been in use before that so it got the bacteria. the goldfish i got today so it is not here it got the black color on finns

I am having a comet (maybe a breed of comet and common), mine has longer tail since the day I've bought it at LFS. Yours look more like common with their shorter tails.

Black markings, I won't repeat that it might be ammonia burns.

And correct me if I am wrong but putting 3 goldies at once in a fish tank (which is not certainly sure to be cycled) is not really let's say, good thing. As far as I understand the ecosystem of the tank has to adapt itself for fish. You bumped into your fish tank 3 fishes at once. :-) I would expect some hikes in ammonia and nitrites. I would suggest checking the water parameters.

I hope you get a water test kit soon to make sure they stay healthy, they look great. They recommend 10 gals of water per goldfish but if they're juveniles, depending on their size, they should be ok in a 20 gal for now. Another way to calc it is 2 gallons of water per inch of goldfish, with a minimum of 10 gallons.